Downing Street hits back at Joe Biden and suggests he doesn't understand its Brexit proposals

Downing Street has suggested US presidential candidate Joe Biden doesn't understand its proposed Brexit legislation after the White House hopeful weighed into the controversy.

On Wednesday night, Mr Biden issued a warning that a post-Brexit US-UK trade deal "must be contingent upon respect" for the Good Friday Agreement in Northern Ireland, and the avoidance of a hard border on the island of Ireland.

The Democratic presidential candidate, who is hoping to beat Donald Trump in November's presidential election, wrote on Twitter: "We can't allow the Good Friday Agreement that brought peace to Northern Ireland to become a casualty of Brexit."

Mr Biden, a former US vice president, posted his tweet in response to a letter from four US congressmen warning of their "grave concern" over the UK government's plans to override provisions for Northern Ireland in the UK's withdrawal agreement with the EU.

The government has admitted its UK Internal Market Bill, if passed, would break international law - but stressed it is "critical" to ensuring the unfettered access for goods from Northern Ireland to the rest of the UK, and to protect the Good Friday Agreement.

Mr Biden's stance contrasted with Donald Trump's Secretary of State Mike Pompeo, who said he had "great confidence" that the UK government "will get this right in a way that treats everyone fairly".

Asked to respond to Mr Biden's comments on Thursday, a Number 10 spokesman said: "We continue to remain absolutely committed to no hard border and no border infrastructure between the Republic of Ireland and Northern Ireland.

"And we will continue to engage with our US partners on a bipartisan basis to ensure that our positions are understood."

Asked if Mr Biden was wrong, the spokesman responded: "We will continue to work with our US partners to ensure our position is understood, but the whole point of this - as the prime minister has set out - is to make sure the Belfast/Good Friday Agreement is upheld."

Downing Street's comments came after former Conservative leader Sir Iain Duncan Smith warned Mr Biden to "worry more" about "killing and rioting" in the US than the government's Brexit legislation.

He told The Times: "We don't need lectures on the Northern Ireland peace deal from Mr Biden.

"If I were him I would worry more about the need for a peace deal in the USA to stop the killing and rioting before lecturing other sovereign nations."

The government's UK Internal Market Bill has prompted a storm of criticism both at Westminster and in Brussels.

On Thursday, the EU stuck by its demand for the UK government to withdraw part of the proposed legislation by the end of this month.

Eric Mamer, chief spokesperson of the European Commission, told reporters: "We have as you know set out a position extremely clearly, it is in our statement, and it relates to those clauses being withdrawn from the law.

"That position has not changed and we have asked the UK to do this at the earliest possible convenience and by the end of September at the latest. That has not changed."

Mr Mamer had been asked whether Prime Minister Boris Johnson's compromise with Tory rebels - which would give MPs a vote before using powers that would break international law - had altered the EU's stance.